Polyurethane is applied in a wide range of fields such as foams, adhesives, paints, elastomers, artificial leather and fibers, and there are many useful polyurethane products.
Among them, a polyurethaneurea elastic fiber, which is especially required to have a high elasticity, usually comprises a segmented polyurethaneurea and is produced by the "two-stage process" comprising the following two stages (1) and (2): (1) First, a diisocyanate component and a diol component are reacted with each other in a molten state to give a molten prepolymer having at both ends thereof an isocyanate group, which is then dissolved in a solvent such as dimethylformamide or dimethylacetamide to prepare a prepolymer solution (melt synthetic process). Alternatively, both the components may be directly reacted with each other in the above-mentioned solvent to prepare a prepolymer solution (solution synthetic process). (2) Then, the prepolymer is subjected to chain extension with aliphatic diamines as disclosed in U.S. Pat. No. 2929804 to prepare a segmented polyurethaneurea solution, and then the solvent is removed from the solution to give a polyurethaneurea elastic fiber. However, since unfavorable side reactions (crosslinking reactions) are liable to occur in the course of the production of the above-mentioned polymer, the resultant final polymer has a high degree of a branched structure, and this renders the polymer solution liable to become highly viscous or gelled. Even though a method wherein the productivity is not taken into consideration is used, for example, a lowering in the concentration of the polymer solution, the liability of the polymer solution to gelation cannot be completely eliminated. Further, a lowering in the solution viscosity occurs due to breaking of the branching in the subsequent step, such as incorporation of a stabilizer, which makes it difficult to stably conduct molding such as spinning due to the presence of a microgel in the polymer. To solve this problem, Japanese Examined Patent Publication (Kokoku) Nos. 41-3472, 44-22311 and 47-35317 disclose methods wherein the branched structure of the polymer is reduced to lower the viscosity to a value suitable for molding. The reduction of viscosity results from the chain scission reaction, which requires considerable time. In some cases, the polymer solution is colored or loses its transparency. Further, the microgel cannot be completely removed, so that yarn breakage still occurs during spinning; for example, when a yarn having a small denier is spun or a spinning rate is increased. It is also possible to prepare a polymer solution having a viscosity suitable for molding through a lowering in the molecular weight or concentration of the polymer, without cutting the branching, and then conduct the molding. In this method, however, it is difficult to avoid a change in the viscosity due to the reduction of branching in the subsequent step, so that a difficulty arises in stably conducting the molding. Therefore, a molded article having high physical properties cannot be prepared, and as mentioned above, the productivity of the polymer solution is also lowered.
On the other hand, an attempt has been made to prepare from the outset polymer having a small degree of branching through the suppression of a branching reaction (a crosslinking reaction). For example, Keiji Iwata "Poriurethan Jushi (Polyurethane Resin)" (The Nikkan Kogyo Shinbun, Ltd.) describes that an acidic substance has the effect of suppressing a crosslinking reaction, when the prepolymer reaction is carried out in a neutral solvent such as toluene or in the absence of a solvent. This is because, we believe, an alkaline compound, which is used as a catalyst for preparing the starting diol component and remains in the starting diol component and which accelerates the crosslinking reaction, is neutralized and deactivated. Contrary to this, in the case of, for example, dimethylformamide (pKa=-0.01) and dimethylacetamide (pKa=-0.18), which are a good solvent for the present segmented polyurethaneurea and a basic solvent, even when a small amount of an acidic substance is added thereto, the action or effectiveness as an acid is decreased. Furthermore, the above-mentioned solvents are decomposed, in the presence of an acidic substance, to an amine and a carboxylic acid and, as a result, the acidic substance added is reacted with the decomposed amine to form a neutral salt. Although the carboxylic acid remains, the carboxylic acid does not substantially act as an acid in the above-mentioned solvents. Thus, in such a basic solvent, it is considered that the acidic substance does not have an effect to inhibit the crosslinking reaction, unlike a solvent (e.g., toluene) which is neutral and which is not decomposed. However, surprisingly, it has been found that, when the prepolymer solution is produced by a solution synthetic process or a bulk synthetic process, as mentioned above, in the above-mentioned basic solvent, the prepolymer is chain-extended with an organic diamine, when a small amount of an acidic substance is previously added, and the branching structure in the resultant polymer is remarkably small. However, in some cases, a polymer solution in a gel form with a high viscosity and a high degree of branching or a polymer solution having a small degree of branching but a remarkably low solution viscosity and suffering from a loss of transparency to be prepared. Consequently, the spinnability of these polymer solutions was unstable, the resultant elastic fiber had low physical properties, and the physical properties were varied to a great extent and instable.
Thus, when a segmented polyurethaneurea solution is produced by preparing a prepolymer solution according to a solution synthesis process or a melt synthesis process, and the prepolymer solution subjected to chain extension, in the prior art, a solution which made possible stable production of providing an article having high physical properties, that is, a homogeneous, transparent solution of a segmented polyurethaneurea having a remarkably small degree of branching in the polymer was not possible.